Depends. They may be sacrificing higher profits by going the GDDR5 route for the sake of looking better than Nvidia for the time being.

I think AMD is going with the better route here by going with GDDR5. While the initial costs of either going with GDDR5 or going with a higher memory interface may be similiar, the costs of GDDR5 will eventually come down as yields go up and with more manufacturing ramp up. A higher memory interface will always hog up the same amount of silicon, dragging down yields on the entire wafer and making each die cost more.

By the time D12U comes around, I wonder what AMD will do to try to one up Nvidia... GDDR6 on 512bit on 40nm?